Heat-insulating structural carbon material and process for producing heat-insulating structural carbon material

ABSTRACT

The invention relates to a heat-insulating structural carbon material and the process for producing thereof. This heat-insulating structural material is prepared from a suspension of discrete carbon fibers in a viscoplastic liquid such as polyglycol, glycerine and petroleum oils, by molding a preform and orienting discrete filaments, followed by baking the preform and depositing pyrolytic carbon in the porous structure thereof. This heat-insulating material comprises a coke matrix, discrete carbon filaments, the coke matrix as a porous film-like structure of coke residue on said filaments, and pyrolytic carbon.

SPECIFICATION

1. Field of Invention

The present invention relates to the field of production of refractorystructural materials and can be useful in the production of heatinsulation for a high-temperature technology, in metallurgy andaerospace engineering.

2. Description of Prior Art

The prior art in the field to which the present invention relates can becharacterized by several conventional technical solutions in one oranother way directed to obtaining porous (low-density) materials,insofar as porous carbon materials have thermal conductivity 10 to 15times as low as that of compact carbon materials.

There is known in the art a process for producing porous heat-insulatingcarbon-based materials by introducing a sponging agent, in particular ametal salt, into the starting components JP, A, 59-141410!. According tothis process, graphite powder is mixed with a binder (synthetic resin orpetroleum pitch) and metal salt powder (NaCl). The resulting mixture ismolded and upon cooking at a high temperature is subjected to leachingthereby dissolving the salt and forming pores.

The disadvantage of this process resides in obtaining a material havingisotropic properties and a rather high thermal conductivity due tothermal radiation within the pores and a high thermal conductivity ofgraphite.

Also known in the art is a process for producing a porous carbonmaterial based on short carbon fibers and a binder with an additive of adissolvable granular substance which leads to the pore formation upondissolving JP, A, 61-50912!. According to this process small lumps of amixture consisting of short carbon fibers and a resin are milled, and tothe resulting composition a dissolvable granular substance is added.Upon heating and molding under pressure the preform is placed into asolvent, and after dissolving the granular substance, is baked(carbonized).

This process makes it possible to obtain highly porous articles having anarrow small pores distribution and a reasonably high mechanicalstrength, yet low heat-insulating properties in spite of the porousstructure and the presence of fibers, insofar as it does not prevent theradiation heat loss.

Heat transfer in a carbon composite is known to occur either as a heatflow across the structure compact part or via reradiation in the pores.The degree of heat transfer by the heat flow across the monolithic(compact) parts of the composite is defined by the heat flowproportionality to the volume fraction of the solid component in the lowdirection. Whereas the intensity of heat transfer via reradiation isproportional to the 4th power of the absolute temperature, i.e. itdrastically increases with temperature. Therefore in case of aheat-insulating material working under high-temperature conditions withvisible radiation being generated, the radiation effect can be reducedby putting shields such as transversely-oriented fibers, in the paththereof.

Further known in the art is a process for producing a low-densityheat-insulating carbon material based on discrete fibers 250 to 750 μmlong and a carbon-containing binder with an additive of flake graphiteU.S. Pat. No. 4,152,482!. The process comprises mixing carbon fibers, abinder (pitch), and milled natural graphite in a ratio of from1.0:0.5:1.0 to 1.0:0.7:0.5 (% by weight) followed by carbonizing thebinder.

The resulting material of 180 to 200 kg/m³ density exhibits thermalconductivity under vacuum of 0.18 to 0.20 W/(m*K) at 1300K and 0.6 to0.8 W/(m*K) at 2500K, yet lacks an adequate load-carrying capacityrequired for structural materials, inasmuch as it is not rigid enough,thus being useful only for filling gaps or requiring additionalstrengthening means.

Still further known in the art is the most closely related to thepresently claimed proposal a process for producing a heat-insulatingcarbon composite material based on carbon fibers and a carbonizedthermosetting binder U.S. Pat. No. 3,793,204!. The process comprisespreparing a suspension of carbon fibers and a powdery thermosettingbinder in a dispersive liquid such as water. The water is then removedby pouring the suspension into a tank with a perforated bottom, theresulting fibrous sediment, as intended, being oriented along the tankbottom while water removing. Thereupon the sediment is dried, thepolymer thermosetting binder is cured, then heat-treated in an inertatmosphere till getting carbonized.

To obtain preforms of more than 3 cm in thickness, the above-describedcycle is repeated several times to prevent the material from buckling.

The use of water as the dispersive liquid according to this processmakes it impossible to properly arrange the carbon fibers, inasmuch aswater possesses poor wettability with respect to the fibers and thebinder particles, resulting in their rapid sedimentation from thesuspension, i.e. the suspension itself is unstable in time. Moreover,when removing water (by suction through the mold perforations), thefibers that are carried by the rapid water flow or sedimentation arebeing oriented to a non-lesser degree in the opposite direction (alongthe perforation axis).

Furthermore, using water necessitates drying the preform layerssedimented, thereby increasing the duration of and labor input into theprocess, while giving no additional improvements to the materialproperties.

Moreover, the coke resulting from the binder (resin or pitch)carbonization has a densified structure in the layers adjacent to thefibers surface, which neither imparts strength to the material, norimproves heat-insulating properties thereof.

And the high content of the binder forming a coke matrix uponcarbonizing decreases the material processability, insofar as thecarbonization in this case gives rise to internal stresses, cracking,considerable gas evolution, and, as a result, changes of geometricaldimensions and shape of articles being produced according to thistechnology.

DISCLOSURE OF INVENTION

The essence of the present invention consists in providing a simple andinexpensive production process which would ensure obtaining alow-density (light-weight) heat-insulating material non-prone toshrinkage under baking and possessing load-carrying structuralproperties, whose heat-insulating characteristics are comparable tothose of the prototype described in the prior art review hereinabove.

The solution of the problem pursued is ensured due to the fact thataccording to the claimed process for producing a structuralheat-insulating carbon material which comprises preparing a suspensionof discrete carbon fibers by stirring them in a dispersive liquid,molding a preform and orienting the discrete fibers, with a portion ofthe dispersive liquid being removed, followed by baking the preform in amold, as the dispersive liquid use is made of a viscoplastic liquidexhibiting a good wettability with respect to the suspension components,the suspension preparation by stirring carbon fibers in a thermoplasticliquid being effected till the fibers are separated into uniformlydistributed filaments and after molding and baking, an additionalrigidity is imparted to the preform by depositing pyrolytic carbon inits porous structure. The orientation of the filaments is preferablyeffected by prepouring the suspension over the Nutsch-filter surface,where the preform preshaping is also effected.

The employment of a viscoplastic liquid (polyglycols, glycerine,petroleum oils, or a mixture thereof) as the dispersive liquid isdirected to attaining a complex of factors essential for theheat-insulating material and the production process thereof, due touseful work of forces resulting from the interaction of the wettingliquid with the carbon fibers and filaments, as well as thecarbonization peculiarities of such a liquid:

preparing a stable suspension of filaments to ensure their smoothsedimentation and orientation parallel to the filtering surface, whenpartially removing the liquid via filtering;

obtaining a laminated structure preform when hydrodynamically orientingthe filaments on the Nutsch-filter;

ensuring stability of the wet preform obtained on the Nutsch-filter;

preventing the material shrinkage when carbonizing the viscoplasticliquid remained in the preform, as well as ensuring low gas evolution;

obtaining upon carbonizing the viscoplastic liquid a coke matrix in theform of a porous film of the coke residue on the filaments andtherebetween.

The above factors ensure preparing a preform of a desired thickness (upto 100 mm) in a one-cycle process of molding and baking, and prevent thematerial from buckling and cracking.

The preparation of the fiber suspension in the viscoplastic liquidmedium able to wet the fibers' surfaces promotes a quick separation ofthe fibers into filaments under the action of wetting forces andformation of the filaments suspension therein actually in a state closeto their zero buoyancy. Furthermore, the wettability of the viscoplasticdispersive liquid favors leaving on the filament surface (upon partiallyremoving the liquid by suction) a viscoplastic liquid film ensuringstickiness of the filaments therebetween and thus a possibility ofhandling them as a single preform, and upon carbonizing the viscoplasticliquid this film is converted into a porous film of a coke residuebinding together the filaments. In this case the coke yield is no morethan 0.15 g from 1 g of the viscoplastic liquid mass, i.e. much lower(over three times as low) than that from pitch of synthetic resin.Besides, due to the above, under carbonization many of the wettedfilaments in the preform tend not to harden at once but rather togradually get thicker, thus remaining plastic till the completecarbonization is attained. This ensures the absence of internal stressesand cracks and, accordingly, makes it possible to increase the preformthickness in a single cycle.

The use as the viscoplastic liquid of a mixture of pitch and oil (oils)selected from the group of petroleum products ensures the wet preformplasticization, especially required when producing articles of intricateshapes (the wet preform being bent while molding). Here pitch isemployed as the plasticizer, becoming such as a result of contact withthe viscoplastic liquid which partially dissolves the pitch. Due to thelow pitch content, the further conversion thereof into a coke matrix isnot detrimental to the material properties.

The preliminary operation of pouring the filaments suspension over theNutsch-filter surface provides (due to the viscoplastic liquidviscosity, mobility of the filaments wetted therewith, and the slowfiltering thereof through the Nutsch-filter perforations) a furthermeans to arrange the filaments carried by the suspension flow along theNutsch-filter surface and to obtain not only the orientation parallel tothe perforated surface, but also a laminated texture of the preformfilaments.

In a material of such a texture (with filaments separated by interlayersof the coke residue porous film) a low thermal conductivity is ensureddue to the radiation shielding effect of the filament layers.

Prior to baking the preform is postmolded to attain the solidphase/liquid phase ratio therein of 1:10 to 1:4 so as to have the cokeresidue after baking as a porous film-like structure distributed betweenthe filaments within the material. Such a structure along with theprimary load-carrying properties imparts compliance to the material forit is non-brittle.

The application of a pyrolytic carbon layer onto the porous preformsurface upon baking makes up for some loss of strength caused by theabsence of a conventional binder, and increases the rigidity of the cokeresidue porous structure without densifying the material.

Accordingly, the complex measures used ensures obtaining a light-weightheat-insulating material possessing a load-carrying capacity and areasonably low thermal conductivity comparable to conventional materialsof the same purpose.

PREFERRED EMBODIMENT OF INVENTION

In order to realize the claimed process and obtain a light-weightheat-insulating material exhibiting a load-carrying capacity, asuspension of discrete carbon filaments in a viscoplastic liquid isprepared. As the viscoplastic liquid use is made of a mixture ofcoal-tar pitch and glycerine which mixture is prepared either directlyprior to mixing the discrete carbon fibers with glycerine orsimultaneously therewith. And the starting carbon fibers are to be notover 50 mm long with the pitch addition (particles not over 1 mm) in theamount of not over 0.5% by weight of the fiber.

The fiber length (not over 50mm) is dictated by the fact that in case oflonger fibers it is impossible to mill them into filaments 0.2 to 0.5 mmlong when stirring the suspension, and no laminated structure is formed.While stirring the suspension components in the viscoplastic liquid,there occurs separation of the fibers into filaments, the latter beingmilled down to 0.2-0.5 mm lengths, as well as a partial dissolution ofthe pitch powder. The glycerine is preheated (343-350K) to obtain anoptimal viscosity of from 50 to 500 cP, and together with the charge ofcarbon fibers and pitch is loaded into a mixer where it is stirred for10 to 20 min until the fibers are separated into filaments and ahomogeneous suspension is obtained; the amount of the fiber charge beingdependent on the size of the article to be produced (plates, cones,cylinders, etc.). For example, to obtain an article in the form of aplate of 500*500*50 mm, the carbon fiber charge is to be 1.4 to 1.5 kg.Should larger articles need to be obtained, the carbon fiber charge isto be accordingly increased, along with the other components content.

The viscoplastic liquid is taken in the amount of at least 45 kg. Thenusing a Nutsch-filter the size and shape whereof are consistent with themold cross-section, there are effected orientation of the carbonfilaments, laminated laying-up thereof, as well as a preform firstformation. To this end the resulting suspension is poured over theNutsch-filter net having a passing cross-section of the perforations notover 1 sq. mm so as to attain a uniform distribution thereof all overthe Nutsch-filter surface, at the same time initiating the glycerinewithdrawal through the Nutsch-filter perforations via evacuation. Thisis possible due to the viscoplastic liquid (glycerine) good wettabilityand viscosity and to the laminar type of the suspension spreading (flow)over the Nutsch-filter net, thereby ensuring a laminated laying-up ofthe filaments.

In case of the viscoplastic liquid viscosity over 500 cP, spreadingthereof is hampered and no laminated laying up is attained. Whereas atthe viscosity under 50 cP a rapid spreading of the liquid takes placewith the laminarity being violated, thereby violating the horizontalfilaments orientation and giving rise to the fiber curling. Theevacuation degree with increasing the wet preform thickness on theNutsch-filter, is steadily increased finally resulting in the absolutepressure under the Nutsch-filter of 0.07 to 0.085 MPa.

Upon spending the suspension full portion, the evacuation is furthercontinued for 3 to 5 min per each millimeter of the preform thickness,up to 10 kg of glycerine (with coal-tar pitch) being retained in thepreform per each kilogram of the carbon filaments. Then the resultingwet preform is removed from the Nutsch-filter without disturbing thefilaments laying-up and put into a mold. The thus obtained wet preformthickness exceeds the designed one, which is necessary to enable thesubsequent postmolding of the preform be effected in order to obtain adesired ration of the solid and liquid phases (from 1:10 to 1:4).

At a higher amount of the liquid phase, the wet preform material has astructure that fails to enable the required geometrical dimensions ofthe article to be obtained; whereas at a lower amount of the liquidphase the filament bonding therebetween is not strong enough, resultingin the formation of a loose structure material without sufficientrigidity.

The preform in the mold is brought down to the design thickness under aspecific pressure of up to 0.7 MPa on the press, thereby removing theviscoplastic liquid excessing amount, whereupon the preform is fixed inthe mold at this thickness and supplied for baking to carbonize thereof.

The preform carbonization is carried out in an inert or reducing gaseousatmosphere, preferably in a coke (carbon) filling medium. In this casethe liquid glycerine and its vapors are trapped by the porous carbonmedium and undergo pyrolysis with destruction to simple volatiles(carbon dioxide and steam).

The temperature rise is effected for 24 h up to 1073-1173K, with holdingat the upper temperature level for not over 90 h. After carbonizing thepreform rigidity is increased by depositing pyrolytic carbon in theporous structure thereof. Despite the fact that after the coke matrixformation the preform is already capable to retain its shape and ismachinable, it is still insufficiently rigid: the strain curve of such amaterial actually has no portion of elastic loading and is of a forcedplasticity nature.

And the pyrolytic carbon deposition onto the inner pores surface of thematerial imparts to the latter elasticity and increased load-carryingcapacity, actually without changing the porosity (apparent density)thereof.

The pyrolytic carbon deposition is conducted in an electric vacuumfurnace in a city fuel gas the main component whereof is methane.Hydrocarbons and other types of gases (propane, butane, benzene, etc.)may also be employed.

Depending on the methane content in the city fuel gas, the charge, andthe furnace space, as well as the temperature of preforms undertreatment the process is carried out at the city gas flow rate ensuringthe acceptable duration (40 to 100 h) to obtain a material having adesired thermal conductivity. Here the amount of pyrolytic carbon beingdeposited is 15 to 32% by weight of the preform. At the pyrolytic carbonamount over 32% the material thermal conductivity increases, whereas atthe amount under 15% the desired rigidity and, accordingly, theload-carrying capacity, are not attained.

To better understand the present invention, given hereinbelow arespecific Examples illustrating the embodiment of the present process forproducing a heat-insulating structural material, as well as the testingresults.

EXAMPLE 1

An article in the form of a plate of 500*500*50 mm was produced. Asuspension was prepared from carbon fibers up to 45 mm long in glycerinewith an additive of pitch powder of not over 1 mm grain size. Thecomponents were taken in the following amounts, kg:

    ______________________________________                                               *Carbon fibers                                                                         1.5                                                                  *Pitch   0.3                                                                  *Glycerine                                                                             45.0                                                          ______________________________________                                    

The resulting mixture was stirred in a paddle mixer for 15 min at thespeed of 2 rps. The glycerine was preheated up to the temperature of348K at which a desired viscosity (450 cP) was attained.

By pouring the resulting suspension over the Nutsch-filter surface, thedesired thickness (by 10% over the design thickness) was graduallyattained, providing under the Nutsch-filter the absolute pressure of0.07 MPa which was gradually decreased with the preform thickness. Thevacuum was further maintained for 5 min. In the resulting (washed-put)preform the solid phase/liquid phase ratio attained was 1:10.

The preform was removed from the Nutsch-filter by turning over thefilter content onto an auxiliary plate.

Upon supplying the resulting preform into a mold, it was compressed at aspecific pressure of 0.7 MPa until the design thickness of 50 mm wasattained.

The preform carbonization was effected in the mold at the temperature of1173K with the temperature rise for 24 h and exposure for 90 h. Thenpyrolytic carbon was deposited onto the pores surface within thepreform. The process was conducted in a city gas (90% methane, 5%hydrogen) in a furnace of 6 m³ space under the absolute pressure(2200±665 Pa) and at the temperature of 1223K. The flow rate of the citygas was maintained at 7.5±0.12 m³ /h.

The resulting material has the following ratio of the components, % byweight:

    ______________________________________                                               *Carbon filaments                                                                        46                                                                 *Glycerine coke                                                                          18                                                                 *Pitch coke                                                                               4                                                                 *Pyrolytic carbon                                                                        32                                                          ______________________________________                                    

The coke in the resulting material was present as a porous film-likestructure distributed on the filaments and therebetween. The cokeresidue surface was coated with pyrolytic carbon.

To determine physico-mechanical properties of the resulting material,there were produced samples of a regular form, mainly in the form of aparallelepiped, by cutting the obtained preforms (articles) with a saw,cutters, or the like, and apparent density, ultimate compressivestrength, ultimate bending strength, thermal conductivity, and ashcontent were measured.

The apparent density was determined on the basis of the samplemass/volume ratio, the volume measurement error not exceeding 0.7%.

The ultimate compressive strength was determined on samples of 20*10*10mm and 10*10*10 mm.

The resulting material structure was visually investigated using amicroscope.

The bending tests (3-point bending) were conducted on samples of100*20*10 mm.

The thermal conductivity was determined on samples of 10*10*10 mm and100*12*10 mm on the basis of experimental data on the temperaturegradient produced by a standard heat source.

The material ash content was determined via calcination thereof in analundum closed-type crucible, in a muffle at the temperature of 1173K inair until the residual constant weight was attained.

The results of measuring physico-mechanical properties of the materialobtained in Example 1 are presented in Table 1.

                  TABLE 1                                                         ______________________________________                                        Characteristics Measurement unit                                                                          Level                                             ______________________________________                                        Apparent density                                                                              kg/m.sup.3  190 to 250                                        Ultimate Strength:                                                            compressive (across                                                                           MPa         1.2                                               the filaments)                                                                bending (along the                                                                            MPa         2.8                                               filaments)                                                                    Thermal conductivity                                                          across the filaments                                                          at the temp-re, K,                                                            in vacuum:                                                                     300            W/(m*K)      0.24                                             2000                         0.60                                             Coefficient of thermal                                                                        *10.sup.-6 deg.sup.-1                                                                     1.2                                               expansion: along and        6.0                                               across the filaments                                                          Ash content     %           0.05 to 0.1                                       ______________________________________                                    

EXAMPLE 2

A plate of 500*500*80 mm was produced. To prepare a suspension,PAN-based high-modulus carbon fibers having the initial length of 40 to50 mm were used in the amount of 2.1 kg. As the viscoplastic liquid usewas made of glycerine.

Upon stirring the mixture of 15 min at the speed of 2 rps, a suspensionfor filaments milled to 0.2-0.5 mm was obtained having a viscosity of400 cP.

Postmolding of a wet preform in a mold was effected under a minimumspecific pressure of 0.4 MPa till the solid phase/liquid phase ratio of1:10 was attained.

Upon carbonizing the preform at 1173K, depositing pyrolytic carbon at1223K, and heat treating at 2373K, a material was obtained whosephysico-mechanical properties are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Characteristics Measurement unit                                                                          Level                                             ______________________________________                                        Apparent density                                                                              kg/m.sup.3  150 to 200                                        Ultimate compressive                                                                          MPa         1.3                                               strength: across the        6.1                                               filaments                                                                     along the filaments                                                           Ultimate bending                                                                              MPa         1.6                                               strength: along the                                                           filaments                                                                     Thermal conductivity                                                                          W/(m*K)                                                       St the temp-re, K:                                                            300, across the fi-          0.22                                             laments                                                                       2000, across the fi-         0.49                                             laments                                                                       along the filaments          0.98                                             Coefficient of thermal                                                                        *10.sup.-6 deg.sup.-1                                         expansion at the tem-                                                         perature of from 300                                                          to 2300 K:                                                                    across the filaments        6.2                                               along the filaments         1.9                                               ______________________________________                                    

The quantitative composition of the material obtained in Example 2 canbe characterized as follows, % by weight:

    ______________________________________                                        *High-modulus carbon filaments                                                                     46                                                       *Glycerine coke      34                                                       *Pyrolytic carbon    20                                                       ______________________________________                                    

The results of measuring thermal conductivity of the material obtainedin Example 2 are presented in Table 3.

                  TABLE 3                                                         ______________________________________                                        Testing  Material                                                             tempe-               Densified with                                           rature,  Carbonized  pyrolytic carbon                                                                          Heat-treated                                 K        Thermal conductivity, W/(m*K)                                        ______________________________________                                         300     0.06        0.22        0.19                                          500     0.07        0.22        0.21                                          800     0.11        0.25        0.29                                         1000     0.16        0.30        0.32                                         1200     0.21        0.33        0.35                                         1400     0.28        0.38        0.40                                         1600     0.37        0.43        0.46                                         1800     0.48        0.51        0.53                                         2000     0.60        0.60        0.62                                         2200     0.76        0.72        0.75                                         2400     0.97        0.97        0.92                                         2600     1.25        1.05        1.10                                         2800     1.66        1.23        1.34                                         ______________________________________                                    

EXAMPLE 3

The procedure of Example 2 was followed, except that rayon-based carbonfibers were used which were subjected to premilling.

The heat treatment after carbonization was conducted at 1873K to obtaina material the properties whereof are presented in Table 4.

                  TABLE 4                                                         ______________________________________                                        Characteristics Measurement unit                                                                          Level                                             ______________________________________                                        Apparent density                                                                              kg/m.sup.3  200                                               Ultimate compressive                                                                          MPa         2.0                                               strength, across                                                              the filamants                                                                 Ultimate bending                                                                              MPa         1.6                                               strength, across                                                              the filaments                                                                 Thermal conductivity                                                                          W/(m*K)                                                       at the temp-re, K:                                                            across the filaments                                                           300                        0.25                                              2000                        1.25                                              Coefficient of thermal                                                                        -10.sup.-6 deg.sup.-1                                                                     2.4                                               expansion at the tem-                                                         perature of from 300                                                          to 2300 K, along the                                                          filaments                                                                     Ash content     %           0.03                                              ______________________________________                                    

The resulting material has the following composition % by weight:

    ______________________________________                                               *Carbon filaments                                                                        48                                                                 *Glycerine coke                                                                          37                                                                 *Pyrolytic carbon                                                                        15                                                          ______________________________________                                    

EXAMPLE 4

The procedure of Example 1 was followed, except that as the viscoplasticliquid use was made of ethylene glycol without pitch addition. A plateof 500*500*45 mm was produced. Carbon fibers were taken in the amount of1.0 kg, and ethylene glycol was used at room temperature (withoutpre-heating) in view of the ethylene glycol low viscosity (about 340cP).

A wet preform was treated on the Nutsch-filter till the thickness of 60mm was attained, whereupon it was postmolded by 25% (down to 45 mm, thesolid phase/liquid phase ratio of 1:4).

Upon carbonizing the preform and depositing pyrolytic carbon, a materialwas obtained the properties whereof are presented in Table 5.

                  TABLE 5                                                         ______________________________________                                        Characteristics Measurement unit                                                                          Level                                             ______________________________________                                        Apparent density                                                                              kg/m.sup.3  250 to 350                                        Ultimate bending                                                                              MPa                                                           strength along the                                                            filaments at the                                                              temperature, K:                                                                300                        2.50                                              1800                        2.80                                              2000                        2.45                                              Ultimate compressive                                                                          MPa         2.0                                               strength across the                                                           filaments                                                                     along the filaments         4.3                                               Thermal conductivity                                                                          W/(m*K)                                                       across the filaments                                                          at the temp-re, K:                                                             300                        0.18                                              2000                        0.70                                              Ash content     %           0.06                                              ______________________________________                                    

The resulting material had the following composition, % by weight:

    ______________________________________                                        *Carbon filaments 51                                                          *Ethylene glycol coke                                                                           26                                                          *Pyrolytic carbon 23                                                          ______________________________________                                    

EXAMPLE 5

A plate of 500*500*50 mm was produced. As the viscoplastic liquid usewas made of petroleum oils (vacuum oils) having a viscosity of up to 10cSt at 327K. 0.45 kg of coal-tar pitch having a particle size of up to1.5 mm were added to 45 kg of the oil.

A suspension was prepared from 1.2 kg of PAN-based carbon fibers 50 mmlong having a viscosity of 200 cP and the solid phase/liquid phase ratioof 1: 9. The remaining properties are presented in Table 6.

                  TABLE 6                                                         ______________________________________                                        Characteristics Measurement unit                                                                          Level                                             ______________________________________                                        Apparent density                                                                              kg/m.sup.3  320 to 340                                        Ultimate compressive                                                                          MPa         1.6                                               strength across the                                                           filaments                                                                     Ultimate bending                                                                              MPa         3.0                                               strength along the                                                            filaments                                                                     Thermal conductivity                                                                          W/(m*K)                                                       at the temp-re, K:                                                            300                                                                           across the filaments         0.22                                             along the filaments          0.55                                             2000                                                                          across the filaments         0.60                                             along the filaments          1.65                                             Coefficient of thermal                                                                        -10.sup.-6 deg.sup.-1                                         expansion in the tem-                                                         perature range of from                                                        300 to 2300 K                                                                 along the filaments         1.8                                               across the filaments        6.2                                               Ash content     %           0.1                                               ______________________________________                                    

The resulting material had the following composition, % by weight:

    ______________________________________                                               *Carbon filaments                                                                        42                                                                 *Petroleum oil coke                                                                      20                                                                 *Pitch coke                                                                              12                                                                 *Pyrolytic carbon                                                                        26                                                          ______________________________________                                    

As follows from the above-presented Examples, the thermal conductivitylevel of the material according to the claimed process is actuallyindependent of the viscoplastic liquid and the carbon fiber types.

And the milling degree of carbon fibers (filaments) in the suspensionprepared, gets actually the same (0.2-0.5 mm) upon stirring regardlessof the initial carbon fiber length (not over 50 mm).

All the samples investigated have the fiber laminated laying-up and thecoke matrix in the form of a filmlike structure.

INDUSTRIAL APPLICATION

The present invention can be useful for producing rigid heat-insulatingstructures, such as ducts, cylinders, lining members, heat-insulatinghousings for various apparatuses both of regular and intricateconfigurations.

We claim:
 1. A process for producing a carbon structural thermalinsulator, and comprising, preparing a suspension of discrete carbonfibers by mixing said fibers while agitating said fibers in a dispersiveliquid, each of said discrete carbon fibers being made of a plurality ofdiscrete filaments, molding a preform and orienting said discretefilaments, eliminating part of said dispersive liquid, and baking saidpreform in a mold, wherein said dispersive liquid is anon-water-containing viscoplastic liquid having a good wettabilitycompared to components of said suspension, during preparation of saidsuspension said carbon fibers are mixed and agitated in saidviscoplastic liquid until the fibers separate into said filaments suchthat said filaments are uniformly distributed by carbonizing saidviscoplastic dispersive liquid in said preform, a porous structure ofcoke residue is formed on said filaments and additional rigidity isgiven to said preform after molding and baking of said preform bydepositing pyrolytic carbon in the porous structure of said preform. 2.A process according to claim 1, wherein said viscoplastic liquid is apolyglycol or glycerin.
 3. A process according to claim 1, wherein saidviscoplastic liquid comprises a petroleum oil.
 4. A process according toclaim 1, wherein said viscoplastic liquid is a mixture of pitch and apolyglycol.
 5. A process according to claim 1, orienting of thefilaments is brought about by pouring beforehand the said suspensiononto a surface of a filter whereon a preform is first shaped.
 6. Aprocess according to claim 1, wherein said suspension is prepared andused in a heated state upon attaining viscosity of 50 to 500 cP.
 7. Aprocess according to claim 1, said suspension is prepared from fibers atmost 50 mm long.
 8. A process according to claim 1, wherein uponremoving a portion of said viscoplastic liquid and preshaping a preform,the latter is postmolded in a mold until a solid phase/liquid phaseratio of 1:10 to 1:4 is attained.
 9. A process according to claim 1,wherein said viscoplastic liquid is a mixture of pitch and at least onepetroleum oil.
 10. A process according to claim 1, wherein saidviscoplastic liquid is a mixture of pitch and a mixture of petroleumoils.
 11. A process according to claim 1, wherein said viscoplasticliquid is a mixture of pitch and glycerin.